A base material for screen printing which comprises a screen, a resist layer comprising photosensitive material and a protective film, is known per se and available from Gallus Ferd. Rüesch A G, under the trade name Screeny Siebdruckplatten. In the specialist field, the screen is also described as a network of dykes which delimit openings. The known screen comprises a woven metal mesh galvanized with nickel. When the base material is used for screen printing, first of all the protective film is removed from the base material. The resist layer is exposed according to a defined pattern, developed and the resist layer is cured in accordance with said pattern. After the unexposed, uncured resist layer has been removed, the base material can function as a stencil during the screen printing. There are also known resist layers which react in the opposite way to exposure, i.e. which cure without exposure and do not cure if they are exposed. Therefore, the negative image of the pattern exposed is cured. The protective film has the function of protecting the resist layer during storage, transportation and when the base material is being rolled up and unrolled. In addition, the film provides the base material with an attractive appearance. The base material can, for example, be made into a cylindrical shape and used in this form for screen printing, in which case—after the protective film has been removed and after the resist layer has been exposed and developed—the ink is guided through the screen using a squeegee. The pattern which is printed in the process is determined by the resist layer which is present on and in the screen.
U.S. Pat. No. 4,705,608 has disclosed a method for producing base material of this type. In one embodiment of this method, a layer of a photosensitive emulsion or solution is applied to a plastic film. Then, mesh which is provided with a covering layer that has been deposited by electroplating in order to join the nodes of the mesh to one another is pressed into the said layer. After that, a second layer of photosensitive emulsion is applied from the uncovered rear side of the mesh and pressed through the mesh in order to join the mesh to the first layer of photosensitive material, and then the unit is dried. These steps are carried out in such a way that the film comes into contact with a cylindrical heated surface. In the method, the film functions firstly as a release film, in order to prevent the first layer of photosensitive material from sticking to the heated surface at the end of the production steps. Then, if the film is not removed, it can also remain in place as a protective film to protect the photosensitive material. In another embodiment, the second layer of photosensitive material is arranged in the space between the film with the first layer on it and the mesh, it being ensured that the material applied last is pressed through the mesh openings towards the rear side. Excess material on this side is then removed with the aid of a scraper element. Instead of the first resist layer being applied to the plastic film, it is also possible to employ a ready-to-use, commercially available film with a layer of photosensitive material, in which case this layer is moistened in order for the mesh to be applied. The product of this known method is a base material in which the photosensitive material is located on both main surfaces (the printing side and the squeegee side).
GB-A-2176630 discloses a screen-printing stencil comparable to the base material described above, in regard of which it is stated that the surface of the photosensitive material has a flat surface. This material can be produced using the method described in the abovementioned US patent, in which case the wires of the mesh must not disturb the flat surface of the photosensitive material.
It has now been found that the extent to which the surface of the resist layer is flat on the side of the protective film nevertheless leaves something to be desired, in view of the requirements that are often imposed: despite the abovementioned requirement imposed on the execution of the method, the surface of the resist layer is visible in the known base material, in the form of a more or less undulating surface, also referred to as a relief. When the base material is used for screen printing, this relief has proven to have an adverse effect on the results of printing, since the printed image is less sharp at locations where the relief is prominent.
Furthermore, if the base material is used as a screen-printing stencil, it may be disadvantageous for both the squeegee side and the printing side to be provided with a resist layer.